Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Rye cover crop used as a mulch.
With cover crop season coming up, vegetable growers will have decisions to make on what cover crops to plant and how best to grow and use them. The following is a repeat of an article revisiting this topic.
Cover crop acreage has been growing in the region, largely due to nutrient management efforts, cost share programs and recent programs encouraging farmers to grow cover crops for soil health benefits and soil improvement.
Nutrient management goals and soil health goals are not necessarily the same. In nutrient management-based cover crop programs, the goals are to have crops that can take up residual nitrogen and provide cover to reduce soil erosion losses. Non-legumes predominate, with most of the acres planted in small grains such as rye and some recent use of radishes. Limited or no fertilizer can be used with cover crops in these programs. In this case the answer to the question above is that a cover is being grown. While there will be soil health benefits, they are not maximized.
In contrast, when soil improvement is the primary goal, the cover crops are grown as crops. You are growing plants to maximize the benefits they provide. To increase organic matter and improve soil health the main goal is to produce maximum biomass above ground and below ground. A second important goal is to provide different types of organic matter (such as with cover crop mixtures) to support a diverse soil microbial environment.
Cover crops can also be important to break soil borne pest cycles. Certain cover crops such as some rye varieties can reduce root knot nematode levels because they are non-hosts. Other cover crops such as hairy vetch can reduce Fusarium levels in watermelon rotations. Cover crops also can be an important part of weed management programs through the production of allelopathic chemicals that inhibit weed seed germination.
In other cases, the goals will be different. With leguminous cover crops a goal may be to maximize the amount of nitrogen fixed. With soil compaction reducing crops such as radishes, the goal is to maximize the amount of “biodrilling” (i.e. the amount of tap roots being produced). With biofumigant crops, the goal is to maximize the production of fumigant-like chemicals by the crops. With mulch-based systems, the goal is to maximize above-ground biomass.
What these soil improvement and specific use goals have in common is the need to treat the cover crop as a crop to optimize plant growth. This includes seeding at the proper rate to achieve optimal stands, planting at the right time, using best seeding methods to get maximum seed germination and plant survival, having sufficient fertility to support good plant growth, providing water during dry periods, managing pests (insects, diseases, weeds), and inoculating legumes. If cover crop mixtures are being used, the ratios of seeds being planted must be considered to have the best balance of plants in the final stand.
The best cover crop stands are obtained with a drill or seeder that places the seed at the proper depth, at the proper seeding rate, with good soil to seed contact. Fertilization and liming programs should be used to support season-long growth – fertilizers and other soil amendments will be necessary in most cases. Nitrogen will need to be added for non-legumes.
When the crop is terminated is also key. The cover crops should be allowed to grow to the stage that maximizes the benefits they have to offer before killing the crops. Allowing a winter cover to grow for an extra week in the spring can make a large difference in the amount of biomass but may cause water deficits in dry years.